The attached image is the rhombus in question.
To start, a rhombus will have perpendicular bisectors.
With that, we know that the section where these bisectors cross will create 90 degree angles.
So now we have angles: x, (x-10), and 90.
A triangle will total 180 degrees in total. So if we add up all of these angles, it will total to exactly 180.
In other words: 90+x+(x-10)=180.
So, in this situation, we want to isolate the variables, and find the value of x.
This is the easy part:
Here's that equation again
90+x+(x-10)=180
Subtract 90 from both sides
90-90+x+(x-10)=180-90
x+(x-10)=90
Add 10 to both sides
x+x-10+10=90+10
x+x=100
2x=100
Divide by 2 on both sides
2x/2=100/2
x=50
There's your x value.